Functional Requirements and Design of Gas Exchangers

Abstract

In nature, the aphorism that ‘necessity is the mother of invention’ has been pertinent to the elaboration of novel biological designs, as it has been to the advancement of human technology. As in architectural and human engineering designs, fabrication of biological structures entails exploitation of the physical properties (e.g. strength and reliability) of the constitutive components. Given specific quantities and kinds of structural materials, infinitely many types of constructions are theoretically possible. However, notwithstanding this possibility, in biology, only a calculable number of design forms eventuate. Constructional, phylogenetic, developmental, functional and ecological constraints prescribe particular phenotypes. These outcomes must be the most optimal ones under the particular circumstances in which they form and given the needs that they are programmed to fulfill. Considering the remarkable morphological diversity of extant animals, the manifested structural (Fig.2) and functional (Fig.3) designs of the gas exchangers represent a very narrow range of the forms that may have evolved and far fewer than those that are theoretically possible. Through remarkable convergence, only a few different designs have developed. This conceivably indicates the importance of respiration as a physiological process for the sustenance of life and hence the superlative endeavours to optimize it. About 99.99% of all the animal species that have ever evolved on Earth are now extinct (e.g. Pough et al. 1989). Notwithstanding the particular cause of their demise, the designs of such animals must be considered to have been failed experiments.

I have become convinced not only that structure determines function, but that functional demand also determines structural design, be it through evolution or by modulation of design features. Weibel (1984)